Recent years have witnessed a sharp increase in information traffic due to widespread use of the Internet. The increase in information traffic is expected to continue as mobile data communications, M2M (Machine to Machine) communications, and the like come into further increasing use. Accordingly, there is an urgent need to further increase capacities of optical fiber communication networks which support those communications. So far, attempts have been made to increase communication capacities through a combination of (i) wavelength-division multiplexing (WDM) which makes simultaneous use of a plurality of different wavelengths (see Non-patent Literature 1), (ii) polarization-division multiplexing (PDM) which utilizes differences in polarization (see Non-patent Literature 2), and (iii) the like.
However, a single-mode fiber (SMF) which supports current optical fiber communication networks suffers nonlinear effects in which a signal pulse deforms (see Non-patent Literature 3) and thermal destruction of the optical fiber (see Non-patent Literature 4), each caused by an increased input power density associated with an increased multiplexing order. As such, limitations of the attempts to increase transmission capacities have been pointed out (see Non-patent Literature 5).
In view of this, great attentions are currently drawn to a next-generation optical communication technology for further increasing the capacities of optical fiber communications. An example of promising next-generation optical communication technology is mode-division multiplexing (MDM) which uses a multi-mode fiber (MMF) (see Non-patent Literature 6). In the MDM, the MMF having a core diameter greater than that of the SMF is used, and light beams of a respective plurality of spatial modes propagating through the MMF are independently subjected to signal modulation. This allows optical transmission capacity to be increased in proportion to the number of spatial modes used. Accordingly, an increase in transmission capacity is expected to be achieved.
Further, as described above, the MMF used in the MDM has a core diameter greater than that of the SMF. This allows preventing an increase in input power density. Accordingly, the MDM is less likely to be affected by nonlinear effects and thermal destruction.
Further, the MDM can be combined with an existing multiplexing technology such as WDM, so that an ultimate optical transmission capacity achieved is a multiplication of a current communication capacity and the number of spatial modes used. Thus, a significant improvement in optical transmission capacity is achieved.
For the reasons above, the MDM is a technology that is attracting great attentions. Meanwhile, in actual use of the MDM, it is essential that a transmitting end and a receiving end have a spatial mode multiplexing technology and a spatial mode demultiplexing technology, since the plurality of spatial modes in the MMF are treated as independent channels.
As conventional spatial mode multiplexing/demultiplexing technologies, a technique that uses a phase plate (see Non-patent Literature 7) and a technique that uses a planar lightwave circuit (see Non-patent Literature 8) have been proposed. The technique using the phase plate is characterized in that demultiplexing can be performed even in a case in which spatial modes spatially overlap with each other. The technique using the planar lightwave circuit is characterized in that loss in multiplexing and demultiplexing is small and a reduction in device size is possible. However, according to the technique using the phase plate, it is necessary to provide a certain number of phase plates which number depends on the number of modes used, and it is also necessary to use a beam splitter in order to perform multiplexing or demultiplexing of signal light. This undesirably adds to size and complexity of a system. Further, the technique using the planar lightwave circuit has a problem that the number of modes used is limited, since it is extremely difficult to handle high-order spatial modes under the current technology.
As a technology that can solve the above problems, there has been proposed an all-optical mode multi/demultiplexer which involves a dynamic multiplex hologram realized with use of a hologram medium (see Patent Literature 1 and Non-patent Literature 9). According to the technology, (i) interference fringes between spatial modes and reference light which propagates at a different angle are recorded in the hologram medium in advance, (ii) multiplex-recording of a record thus obtained is carried out while changing an angle between the spatial modes and the reference light so as to obtain a volume hologram, and (iii) the plurality of spatial modes are demultiplexed from the volume hologram in an all-optical manner. This allows a plurality of spatial modes to be handled simultaneously with use of a single device. Further, since the demultiplexing is carried out via recording of the spatial modes in the hologram medium, the demultiplexing can be carried out even in a case where high-order spatial modes are involved.